1. Field of the Invention
This invention relates to an outboard motor steering system.
2. Description of the Related Art
In almost all outboard motors, the outboard motor comprises an outboard motor main unit having an internal combustion engine and a propeller powered by the engine, and is mounted on a boat through a mounting unit having a swivel shaft (steering shaft) and a swivel case that accommodates the swivel shaft rotatably such that the outboard motor main unit is steered.
The swivel shaft is ordinarily rotated by human power, such as the tiller handle type used to turn the rudder by manually operating the tiller handle connected to the swivel shaft and the remote control type used to remotely operate a link mechanism connected to the swivel shaft through a push-pull cable in response to rotation of a steering wheel manipulated by the operator.
Since human-powered steering systems are disadvantageous because they tend to have an unpleasant steering “feel” owing to, for instance, heavy steering load, as taught in Japanese Laid-Open Patent Application Sho 62 (1987)-125996 ('996), an add-on mechanism constituted as a separate unit from the outboard motor and used to power-assist the turning of the steering wheel is known. This mechanism typically includes a steering actuator such as a hydraulic cylinder placed on the boat to power-assist the steering through a link mechanism. The add-on steering system using such an actuator also has disadvantages, most notably that it adds to the number and weight of the components, and that it takes up space on the boat.
Attempts have been made to overcome these drawbacks. Japanese Laid-Open Patent Application No. Hei 2(1990)-279495 ('495), for example, teaches a steering system including a steering hydraulic cylinder that is not attached to the boat, but directly attached to the mounting unit (more specifically its output end (piston rod) is connected to the swivel shaft or the outboard motor main unit, whilst its main body (cylinder) is connected to the swivel case), thereby minimizing increase in the number and weight of the constituent components and saving space.
Aside from the above, the outboard motor main unit may sometimes vibrate due to engine operation and resistance exerted on the propeller. It is therefore preferable to attenuate the vibration to be transmitted from the outboard motor main unit to the mounting unit so as to reduce vibration to be further transmitted to the boat. In view of this, it has been proposed to install a rubber member (as a vibration attenuator) at a portion connecting the outboard motor main unit and the mounting unit, thereby enabling the outboard motor to displace relative to the mounting unit such that the vibration to be transmitted from the outboard motor to the mounting unit attenuates, as taught in Honda Motor Company Service manual of Honda Outboard Motor BF115A/BF130A; pages 12–47 and 13–5; published on May, 1998.
As mentioned with reference to '996, the outboard motor main unit and the mounting unit are generally made displaceable to each other. Accordingly, as suggested by '495, if the actuator output end (or main body) is connected to the outboard motor main unit, whereas the actuator main body (or output end) is connected to the mounting unit, in other words, if the steering actuator is interposed at any location between the outboard motor and the mounting unit, excessive stress may act on the connection from the outboard motor main unit to the mounting unit via the actuator.
On the other hand, as suggested by '495, when the actuator output end is connected to the swivel shaft, whereas the actuator main body is connected to the swivel case, in other words, when the connection of the actuator is completed at the mounting unit by connecting the main body to the swivel case, this can avoid the disadvantage mentioned above. However, this also has disadvantage that its structure degrade the degree of freedom of installing position of the actuator.